Disc brake levers softening after bumpy race

Hey mechanics out there

I recently did a pretty bumpy race where after I went they the bumpy section, my disc brake levers got super soft to the point where the levers would depress all the way to the handlebars without really biting the rotors.

Obviously I have air in the system, but my brake had been working great for the last couple of months before.

Any tips here on how to make sure that doesn’t happen again? I’ve re bled the brakes and they are back to normal, but am concerned obvi on it happening in another race.

The rear was worse than the front brake, but figured the longer hose allowed for more air to get in. Not sure why the bumps would cause this.

Without sufficient fluid in the master cylinder reservoir, which means more potential for air there, the air could be pulled/pushed into the line leading to the soft feel.

The bumps could exacerbate the issue if the master cylinder is low, because the vibration could lead to bubbles and likelihood for the air to get in the lines.

Both together are a recipe for issues. The solution in general is to have a properly blend system, that is maintained on a regular basis (yearly or more often depending on use).

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Any tips when you bleed on how to get more fluid into the master cylinder?

  • Keep the hose as vertical as possible when bleeding
  • Avoid any loops where air bubbles can get trapped
  • Tap any tight curves in the hose with the handle of a screw driver
  • When reinstalling the bleed port screw on the lever (master cylinder), top it off with a drop or two of brake fluid
  • Make sure the the bleed port on the caliper is tightened sufficiently (but don’t strip it)